they have great durability (6 bolt mains, well designed block, splayed, larger ## head bolts), they have great stock cylinder head design that almost matches aftermarket ford cyl heads, and they are very efficient in using the combustion chamber. Im not saying they don't have their small problems (like piston slap), but the stock design is very competitive with a SBF with boltons. if the motor was $5K I'd say no, but these motors are being sold turn-key, 320 hp for $3K with electronics to get them running with efi. hot rod mag did a buildup of a 6L LQ4 motor (escalade motor) and put a hot cam in it and it made something like 410 hp. They are cool motors in their stock form. in built form, the edge gets very narrow
are they good motors? yes. are they good for your early mustang? your call